1. Field Of The Invention
This invention relates generally to electronic oscillating circuits and more particularly to frequency shift keying oscillators wherein the frequency of the oscillator is discretely controlled by a digital signal.
2. Description of Related Art
Oscillators and the modulation of frequencies for frequency shift keying (FSK) transmission of digital data signals is well known in the art. To review a general form of an oscillator circuit of the prior art, refer FIG. 1. A three terminal amplifier Av has its first input terminal 1 connected to the junction of impedances Z.sub.1 and Z.sub.3. The output terminal 3 is connected to the impedances Z.sub.2 and Z.sub.3, and to the output signal terminal V.sub.o. The output signal terminal V.sub.o will be connected to external circuitry. The topology of this structure allows for a series voltage feedback that will allow the circuit to oscillate at a frequency dependent on the impedances Z.sub.1, Z.sub.2, and Z.sub.3.
It can be shown that the open loop gain A of this circuit is: ##EQU1## Where: A.sub.v is the gain of the amplifier Av.
Z.sub.L is the parallel combination of impedances Z.sub.2, and PA0 Z.sub.1, and Z.sub.3 in series. PA0 R.sub.o is the output impedance of the amplifier Av.
Further it can be shown that the feedback factor .beta. is: ##EQU2##
And the loop gain then becomes: ##EQU3## If the impedances Z.sub.1, Z.sub.2, and Z.sub.3 are pure reactances, that is, if Z.sub.1 =jX.sub.1, Z.sub.2 =jX.sub.2, and Z.sub.3 =jX.sub.3, then: ##EQU4## For the loop gain --A.beta. to be real (zero phase shift) then EQU X.sub.1 +X.sub.2 +X.sub.3 =0 Eq. 4b
and ##EQU5##
In order for the circuit to oscillate the loop gain must be positive with at least unity magnitude. This means that the reactances X.sub.1 and X.sub.2 must either be both inductive or both capacitive and the reactance X.sub.3 must be of the opposite sign. That is, if the impedances Z.sub.1 and Z.sub.2 are capacitors then impedance Z.sub.3 must be an inductor, or if the impedances Z.sub.1 and Z.sub.2 are inductors then impedance Z.sub.3 must be a capacitor. If the reactances X.sub.1 and X.sub.2 are capacitive and the reactance X.sub.3 is inductive, the oscillator has a topology referred to as a Colpitts oscillator.
Refer now to FIG. 2 for a review of a practical Colpitts oscillator of prior art. The amplifier Av of FIG. 1 is formed by the NPN bipolar junction transistor (BJT) Q.sub.1 and resistors R.sub.B and R.sub.E. The resistor R.sub.B provides a bias current to keep the transistor Q.sub.1 in a conducting state, while the resistor R.sub.E and the transistor Q.sub.1 develop the output signal at the output terminal V.sub.o.
The impedance Z.sub.1 of FIG. 1 is formed by the series combination of the surface acoustic wave resonator SAWR and the capacitor C.sub.3. The parallel combination of the inductor L.sub.1 and the series combination of the capacitors C.sub.1 and C.sub.2 form the impedance Z.sub.2. The impedance Z.sub.3 of FIG. 1 will be formed by the parasitic capacitance from the base to the collector of the transistor Q.sub.1.
To achieve oscillation, the resonant frequency F.sub.r of the impedance Z.sub.2 should be near the frequency of resonance of the SAWR. The frequency of resonance for the impedance Z.sub.2 then is given by: ##EQU6##
The equivalent circuit of the SAWR is shown in FIG. 3. The frequency of oscillation will be the series resonance frequency F.sub.s and the parallel resonance frequency F.sub.p of the SAWR. The series resonance frequency is given by: ##EQU7## And the parallel resonance frequency is given by: ##EQU8##
The series capacitor C.sub.s is generally very small (on the order of 2 fF), while the parallel capacitor C.sub.p will be several orders of magnitude larger (on the order of 2 pF). By placing a capacitor in series with the SAWR, the series resonant frequency F.sub.s can be changed by a small amount. The capacitor is in series with the SAWR and will allow a deviation sufficient to permit the transmission of digital data by frequency shift keying.
Referring back to FIG. 2, the capacitor C.sub.3 is placed in series with the SAWR to establish a base frequency. In order to change the frequency of the oscillator, a voltage variable capacitor or varactor C.sub.D is placed in parallel with the capacitor C.sub.3. The varactor C.sub.D is a specially designed diode that is reversed biased. As the voltage at the point A increases, the barrier capacitance of the reversed biased diode decreases. The variable resistor R.sub.D establishes the reverse bias voltage at point A. The FSK data signal is coupled through the capacitor C.sub.i and the resistor R.sub.i to modify the voltage and therefore the capacitance of the varactor diode C.sub.D.
This changing of the voltage at node A will allow the Colpitts oscillator to transmit a high frequency signal that will be modulated by the digital data signal FSK data.
The data output signal V.sub.o will be connected to an external circuit such as an antenna matching network to transmit the modulated signal.
The varactor, in general, is difficult to incorporate into an integrated circuit. Additionally, the varactor requires large change in voltage to achieve a reasonable change in capacitance needed for FSK modulation.
Refer now to resistor R.sub.B of FIG. 2. The resistor R.sub.B provides the bias current necessary to keep the transistor Q.sub.1 in a conducting state. This will allow the transistor Q.sub.1 to act as the amplifier for the Colpitts FSK oscillator. However, during periods of non-operation the biasing current of the transistor Q.sub.1 causes additional power dissipation. Additionally the collector current of transistor Q.sub.1 in practical integrated circuits will be on the order of 2-4 milliamps. This will force the resistor R.sub.B to have a resistance of from 10K-100k ohms. Good quality resistors of this magnitude require large areas on an integrated circuit. (Good quality resistors are those resistors whose values can be well controlled and that are not affected by variations in temperature and variations in the power supply voltage source.)
U.S. Pat. No. 4,618,966 (Stepp, et al.) discloses a technique for modulating a carrier wave to one of M different output frequencies. The carrier wave will be varied by a discrete frequency determined by a clocked digital signal having n bits. This will have the number of frequencies M equal to 2.sup.n. The individual frequencies are created and combined with the carrier wave to create a constant amplitude signal.
U.S. Pat. No. 5,550,505 (Gaus, Jr.) and U.S. Pat. No. 3,991,389 (Dwire, et al.) each describe FSK demodulators. The FSK demodulator will detects the incoming FSK signal and extract the encoded binary signal.